In recent years, an organic electronic device using an organic compound as a semiconductor material has been showing remarkable development. Representative examples of its application include: an organic electroluminescence device (which may hereinafter be referred to as “organic EL device”) expected as a new-generation flat panel display; an organic thin-film transistor (which may hereinafter be referred to as “organic TFT”) that has been attracting attention because the transistor enables the production of a thin-film transistor to be used for, for example, the driving of a pixel of a display by a low-cost process such as printing and can correspond to a flexible substrate; and a photovoltaic device (organic thin-film solar cell) as a light-weight, flexible power source.
In general, a high-temperature process and a high-vacuum process are essential for the formation of a semiconductor device using silicon that is an inorganic semiconductor material into a thin film. The high-temperature process is needed and hence silicon cannot be formed into a thin film on a plastic substrate or the like. Accordingly, it has been difficult to impart flexibility to a product into which the semiconductor device is incorporated or to reduce the weight of the product. In addition, the high-vacuum process is needed, and hence an increase in area of the product into which the semiconductor device is incorporated and the reduction of a cost for the product have been difficult.
The use of an organic compound as a semiconductor material has been expected to realize a low-price device because the organic compound can be easily processed as compared with silicon that is an inorganic substance. In addition, various substrates including a plastic substrate can be applied to a semiconductor device using the organic compound because the device can be produced at low temperatures. Further, the semiconductor material made of the organic compound is structurally flexible, and hence the combined use of the plastic substrate and the semiconductor material made of the organic compound has been expected to realize applications to organic semiconductor products taking advantage of such characteristics, e.g., devices including flexible displays such as an organic EL panel and electronic paper, liquid crystal displays, information tags, and large-area sensors such as an electronic artificial skin sheet and a sheet-type scanner.
An organic semiconductor material to be used in any such organic electronic device has been required to improve the luminous efficiency of an organic EL device, to lengthen its lifetime, and to reduce the voltage at which the device is driven, to reduce the threshold voltage of an organic TFT device and to increase a charge mobility for, for example, increasing its switching speed, and to improve the photoelectric conversion efficiency of an organic thin-film solar cell.
For example, a host material that serves to transport charge in a light-emitting layer is important for improving luminous efficiency in a material for an organic EL device. Typical examples of the host materials proposed include 4,4′-bis(9-carbazolyl) biphenyl (hereinafter referred to as “CBP”) as a carbazole compound disclosed in Patent Literature 1 and 1,3-dicarbazolyl benzene (hereinafter referred to as “mcP”) disclosed in Non Patent Literature 1. When CBP is used as a host material for a green phosphorescent light-emitting material typified by a tris(2-phenylpyridine) iridium complex (hereinafter referred to as “Ir(ppy)3”), the injection balance between charges is disturbed because CBP has the characteristic of facilitating the delivery of holes and not facilitating the delivery of electrons. Thus, excessively delivered holes flow out into an electron-transporting layer side, with the result that the luminous efficiency from Ir(ppy)3 lowers. Meanwhile, mCP shows a relatively good light-emitting characteristic when used as a host material for a blue phosphorescent light-emitting material typified by a bis[2-(4,6-difluorophenyl)pyridinato-N, C2′](picolinato)iridium complex (hereinafter referred to as “FIrpic”), but is not satisfactory in practical use particularly from the viewpoint of durability.
As described above, a host material in which injecting/transporting characteristics for both charges (a hole and an electron) are balanced is needed for obtaining high luminous efficiency in an organic EL device. Further, a compound that is electrochemically stable, and has high heat resistance and excellent amorphous stability has been desired, and hence additional improvements have been required.
In addition, among materials for organic TFT devices, an organic semiconductor material having charge-transporting property comparable to that of amorphous silicon has been reported in recent years. For example, the same level of charge mobility as that of the amorphous silicon has been reported in an organic TFT device using, as an organic semiconductor material, pentacene that is a hydrocarbon-based, acene-type, polycyclic aromatic molecule in which five benzene rings are linearly fused introduced in Non Patent Literature 2. However, the use of pentacene as an organic semiconductor material for an organic TFT device is disadvantageous from the viewpoints of an increase in area, flexibility, a reduction in weight, and a reduction in cost because an organic semiconductor thin-film layer is formed by a deposition method in an ultrahigh vacuum. In addition, Patent Literature 2 proposes a method of forming a pentacene crystal in a dilute solution of o-dichlorobenzene without employing a vacuum deposition method, but the production method is difficult and hence a stable device has not been obtained yet. The fact that the hydrocarbon-based, acene-type, polycyclic aromatic molecule like pentacene has low oxidation stability has also been pointed out as a problem.
In addition, researches on an organic thin-film solar cell have been initially progressed on the basis of a single-layer film using a merocyanine dye or the like. However, since the discovery of the fact that the formation of a multilayer film having a p layer for transporting a hole and an n layer for transporting an electron improves the efficiency with which optical input is converted into electrical output (photoelectric conversion efficiency), the multilayer film has been going mainstream. Materials used at the initiation of an investigation on the multilayer film were copper phthalocyanine (CuPC) for the p layer and peryleneimides (PTCBI) for the n layer. Meanwhile, in an organic thin-film solar cell using a polymer, researches have been conducted mainly on the so-called bulk heterostructure in which a conductive polymer is used as a material for the p layer, a fullerene (C60) derivative is used as a material for the n layer, and the materials are mixed and heat-treated to induce micro-layer separation, thereby increasing a hetero interface and improving the photoelectric conversion efficiency. Material systems used here were mainly a poly-3-hexylthiophene (P3HT) as a material for the p layer and a C60 derivative (PCBM) as a material for the n layer.
As described above, little headway has been made in a material for each layer of an organic thin-film solar cell since the early days, and a phthalocyanine derivative, a peryleneimide derivative, or a C60 derivative has still been used. Therefore, with a view to improving the photoelectric conversion efficiency, the development of a novel material that replaces those conventional materials has been earnestly desired. For example, Patent Literature 3 discloses an organic thin-film solar cell using a compound having a fluoranthene skeleton but the cell does not provide satisfactory photoelectric conversion efficiency.